We have performed an iron K-edge XAS study of the core dimensions of the Fe3S4 cluster in P. furiosus three-iron ferredoxin in both oxidized [Fe3S4]0 and reduced [Fe3S4]- forms. The mean Fe-S distances of the oxidized and reduced cluster are 2.24 and 2.27 E, respectively, and the mean Fe-Fe distance is 2.67 E for both oxidation states. We conclude that there are only subtle structural differences between oxidized and reduced clusters in P. furiosus three-iron ferredoxin. This conclusion is in contrast with a recent crystallographic analysis of the closely-related cluster of Azotobacter vinelandii ferredoxin I.